There is a conventional inverter utilizing synchronous rectification, that is, an inverter that performs a rectification process by synchronizing the operation of a rectifier switch (rectifier element) with the operation of a main switch (for example, Patent Literature 1). Such an inverter utilizing synchronous rectification brings about the advantage that the power conversion efficiency can be improved by causing the rectifier switch to operate with a voltage lower than the on-voltage for a freewheeling diode. Note, the on-voltage for the freewheeling diode is a voltage required to turn on the freewheeling diode.
FIG. 26 shows an overall structure of a load driving system that includes an inverter utilizing synchronous rectification.
A load driving system 900 includes a direct current power supply DC, a smoothing capacitor 902, an inverter 901, and a three-phase alternating current (AC) motor 903 as the load.
The inverter 901 has a three-phase bridge circuit composed of a U-phase arm 904u, a V-phase arm 904v and a W-phase arm 904w, which are connected in parallel. The inverter 901 also has a gate drive circuits GD 91 and GD 92, which control switching operations by switches included in each arm.
Since the arms 904u, 904v and 904w have the same structure, the following describes the W-phase arm 904w only. The W-phase arm 904w includes a high-side switch H9 and a low-side switch L9, which are connected in series. In order to achieve synchronous rectification, the switches H9 and L9 are each constituted by a power semiconductor element with a channel region that is conductive in both forward and reverse directions in the on-state. A representative example of such a power semiconductor element is a metal-oxide-semiconductor field-effect transistor (MOSFET).
A gate terminal of the switch H9 and a gate terminal of the switch L9 are respectively connected to the gate drive circuits GD91 and GD92. The operations of the switches H9 and L9 are controlled due to gate drive signals SgH9 and SgL9, which are output from the gate drive circuits GD91 and GD92, being input to the gate terminals of the switches H9 and L9.
A freewheeling diode DH9 is connected antiparallel between the source and the drain of the high-side switch H9, so that the input/output direction of the freewheeling diode DH9 is the reverse of the input/output direction of the high-side switch H9. In a similar manner, a freewheeling diode DL9 is connected antiparallel between the source and the drain of the low-side switch L9. The freewheeling diodes DH9 and DL9 are provided to secure a path through which a current freewheels when, for example, both of the switches H9 and L9 are in the off-state. Patent Literature 1 discloses a technique in which a bipolar diode region that is conductive only in the reverse direction is used as a freewheeling diode DL9. Note, such a bipolar diode region inherently exists in the structure of MOSFET that constitutes a switch. This structure does not require providing a diode separately from the MOSFET, and therefore offers the advantageous effect of reducing the size of each switch. Such a bipolar diode region that inherently exists in the structure of MOSFET is also referred to as a body diode or a parasitic diode.